Sikorsky S76 main rotor mast problem?

[Cyclic Hotline]

Just been hearing about the Sikorsky S76 main rotor mast problem.
Crack found on S76 mast with less than 500 hrs TT. Circumferential "barber-pole" crack around 45% of the mast, discovered by chance whilst performing other, non-related maintenance.
All aircraft require immediate fluorescent dye-penetrant inspection, plus recurring check every 20 hours.

A few years ago an S58T logger suffered a total mast failure and the head went whizzing off. Amazingly the pilot survived with minor injuries.

Any more information on this?

[helidrvr]

The good folks at Sikorsky confirm that the crack was discovered and a service bulletin issued. The FAA has issued an emergency AD which can be read at http://av-info.faa.gov/ad/SW00/002351.htm

Cheers http://www.pprune.org/ubb/NonCGI/cool.gif

[offshoreigor]

Re: Cyclic Hotline

The check is now carried out after every 10 landings.

Cheers

OffshoreIgor

[magbreak]

OffshoreIgor, that was version A. They are now onto version B which is the version mentioned above i.e every 20 flying hours, or 80 landings. The mast must also be inspected visually before and after flight.

[This message has been edited by magbreak (edited 09 November 2000).]

[eurocopter]

Rumour has it the aircraft in question had a very hard deck landing a week before the crack was discovered. The landing was hard enough for the front oleo to puncture the helideck. It seems the AD is out until the crack has been proved to be linked to the landing.

[Cyclic Hotline]

Thursday November 09 06:43 PM EST
Medical Helicopters Grounded Over Safety Worries

TRENTON, N.J. ( APBnews.com) -- Concerned about potential metal fatigue on the state's three emergency medical helicopters, the New Jersey State Police today confirmed it is grounding the aircraft.

"We have 31 pilots and crew members plus the people that are being flown and we're not taking any chances," said Lt. Al Della Fave, a spokesman for the state police.

The state police, which owns three Sikorsky S-76 B helicopters, said they grounded the choppers last week as a precautionary measure after Sikorsky issued an advisory warning of the possibility that cracks might develop on the main rotor shaft.

The advisory -- which was also filed with the Federal Aviation Administration -- covered all S-76 models, the S-76 A, the S-76 B, and the S-76 C, said William Tuttle, a spokesman for Sikorsky Aircraft.

Company: no reports of flaws

The company is researching the problem, trying to determine whether the potential problem exists for all S-76 helicopters or whether it involves specific models or even specific units manufactured at a specific time, Tuttle said.

In the meantime, the company advised S-76 owners to scan the shaft for tiny cracks and to repeat the scan after every 80 flights or 20 hours of flight time, Tuttle said.

So far, Tuttle said, the company has received no reports from the 500 S-76 owners worldwide indicating that any cracks have been found. Worldwide, about 20 of the aircraft are used as airborne ambulances, Tuttle said.

Pennsylvania, N.Y. police step in

In New Jersey, the state police decided to keep the three aircraft on the ground until Sikorsky finishes its research and gives the S-76 B model a clean bill of health, Della Fave said.

Until they do, Della Fave said, emergency medical flights in New Jersey will be done by pilots from the Pennsylvania and New York state police who use aircraft made by other manufacturers as hovering emergency service units.

"It's not like we won't be covered," Della Fave said.

The New Jersey State Police so far is the only agency that has formally grounded the aircraft, Tuttle said.

The New York State Police, which uses two S-76s for a variety of non-medical missions, has pulled its aircraft out of service briefly to check for possible cracks, said Lt. Jamie Mills, a state police spokesman. If no problems are found, those helicopters could be back in service by the end of the day.

By Seamus McGraw, an APBnews.com staff writer.

[oleloggerhead]

to eurocopter:
the helicopter in question did not experience a hard landing! it landed on a wooden heliport offshore. the nose gear touched down on the end of one of the planks and as stuff happens the particular end of the plank was not supported underneath the wood by steel. the pilots kept pitch and tourque on the powertrain while platform personel cut the end of the plank that was sticking up almost in in contact with the main rotor blades. only minor damage to the nose gear doors and only other minor little damage was discovered. by all means remember one thing and that is these transmissions are rated for alot more tourque than the C-30 engines can put to them IMHO.

[Hoverboy]

After having flown S76 for the last 10+ years I've recently encountered a difference of opinion in regard to the daily servo interlock check. My procedure has been: On the odd day of the week Captain's #1 off - #1 servo light on, Co-pilot selects #2 off, no change in #1 light. Captain to center with servo light switching from #1 to #2 then the Captain selects #1 off but the light should remain on for #2. Center both.
Does this not check both sides of the servo interlock system? I'm not able to take it away from the co-pilot and he's not able to take it away from me. That's the whole point of this check, right?
On even days the Captain side starts with #2 and the co-pilot flips his to #1.
The Flight Check Procedures Manual has a much more thorough and involved check of all possible switch positions, but I understood that the more thorough check was used after maintenance and not as a daily Post Start/Pre Taxi check.
Anybody with a clear grasp of the working of this system that might be able to shed light on this issue?
Are other pilots using the same method as me or...?
Thanks!

[This message has been edited by Hoverboy (edited 10 November 2000).]

[offshoreigor]

Re: Hover Boy

Your absolutley right. That's the most efficient way of checking the interlocks. Most people just don't understand the system and because they were 'taught that way' it naturally causes them to impart the wrong info on to their CoJo's.

I've heard people claim that there are four seperate interlocks in the system, i.e. #1/#2 CoJo and #1/#2 Capt. They need to review the system schematic.

HB, you understand the system perfectly.

Cheers,

OffshoreIgor http://www.pprune.org/ubb/NonCGI/eek.gif




[This message has been edited by offshoreigor (edited 10 November 2000).]

[widgeon]

latest release of AD says they found 2 others cracked , one during the manufacturing process.

[Lu Zuckerman]

I may be completely out of line and possibly out of touch relative to the S76 hydraulic servo system but this was the Sikorsky design philosophy when they first started to use dual hydraulic systems.

There was one switch and if it was in the neutral position both hydraulic boost systems were operational.

If the pilot were to turn off the primary system the control voltage had to pass through a pressure switch on the Aux system. If the Aux system pressure switch indicated proper pressure the voltage would then flow to the primary system shut-off solenoid.

If the pilot elected to turn off the Aux system the same would be true. The control voltage had to pass through a pressure switch on the primary system and if that system were under pressure the voltage would flow to the Aux system solenoid shut-off valve. the system architecture was such that both systems could not be shut off at the same time.

That's the way the system was designed but Sikorsky made a really big mistake, at least on the S-58. The pressure switch for the primary system and the shut-off solenoid for the Aux system were right next to each other. The same was true for the pressure switch for the Aux system and the solenoid shut-off valve for the primary system.

The electrical connectors were not clocked which means that the switch connector and the solenoid connectors could be swapped as at that time they were not color coded.

In this case, they were cross connected. When the pilot made his ground check he switched one system off and it went off. When he switched the other system off, they both went off and the cyclic started rotating in a circle at about 250 rpm following the main rotor. It put the pilot in the hospital and for quite some time, he was singing high soprano

------------------
The Cat

[This message has been edited by Lu Zuckerman (edited 11 November 2000).]

[offshoreigor]

Not to be disrespectful Lu :

We were not talking about the overall system but the S76 interlock check. If you were to ask 10 different people, you would get 10 variations on the check. I think HoverBoy just wants to see how many variations on the check are being used in the industry in general.

The interlocks, when functioning correctly will prevent the NFP from turning off a Hydraulic system if the FP switch has already turned one off i.e. #1 Hyd on the Pilot's Collective off, the Co-Pilot cannot turn off the #2 System and visa versa.

Next, when you refer to AUX/PRI, that does not come into play here. AUX/PRI is found on the S61 and they are two entirely different systems. The AUX on a 61 basically controls the 4 servo's in the broom closet,sort of a power assist, while the PRI is directly related to the 3 Main Servos on the Transmission area, directly to the flight controls.

The S76 has two virtually identical Hydraulic systems as a built in redundancy. The only variation is the #2 system which also supplies your utility hydraulics, i.e. landing gear, VTA etc. The #1 system also incorprates the T/R shut-off.

Basically it is impossible to turn off both hydraulic systems at the same time. Following a hydraulic problem which requires the pilot to shut down one system, if there were a subsequant DC Essential Bus failure, the problem system would come back on-line.

Hope this clears things up a bit.

Cheers, OffshoreIgor http://www.pprune.org/ubb/NonCGI/eek.gif

[Cyclic Hotline]

Aberdeen Press and Journal.
Warning of fault grounds copters

A North sea flight operator has grounded two helicopters after cracks were found in the main rotor shaft of a model in the United States.

The move follows a warning from the Federal Aviation Authority in the US that cracks had been found in a Sikorsky S-76 helicopter there.

Scotia Helicopters, which operates nine Sikorsky S-76s, grounded two of its Aberdeen-based aircraft after receiving the notification of potential metal fatigue from the manufacturers.

The problem is believed to have been caused by a faulty batch of parts.

Last night, a Scotia spokesman said: "This was purely a precautionary measure – the main rotor shaft is regularly replaced but the two helicopters had parts from the same batch as the faulty one in America.

"There has been no disruption to our service as replacement helicopters have been brought in from Norway and we expect to have both the grounded ones back in operation by next weekend." There are about 20 Sikorsky S-76 helicopters operating in Britain, including one attached to the Queen's Flight.

They are primarily used for transferring oil workers offshore and for executive charter in other parts of the country.

Investigations are ongoing into the incident but it is thought that it will be limited to seven aircraft in total, five of which operate in Europe.

The company is advising owners to scan the shaft for small cracks and to repeat the safety check after every 80 flights or 20 hours of flight time.

Bristow Helicopters, which operates one Sikorsky S-76 out of Aberdeen, have not been affected.

A company spokesman said: "None of Bristow's helicopters have been affected by this problem. We operate a number of S-76s in the southern North Sea but all are operating normally.

"The faulty batch of parts was very small and has only affected a very limited number of aircraft worldwide," the spokesman stated.

[Lu Zuckerman]

To Cyclic Hotline

Dear Cyclic,

The following was lifted from A Mystery For The Cat posted by Mriya225.

Another point that effects the reliability of components is a condition known as batch sensitivity. This means that if more than one item of the same design is exposed to the same manufacturing processes at the same time. If one is found defective in service another item that was processed at the same time could have the same defect. This is especialy true for springs or any other item that requires heat treating and vacuum degassing. Another thing effecting springs is the wire draw process which can introduce inclusions which can weaken the spring. This inclusion can be in only one spring or it could be in all springs that were made from that same wire draw. Springs that are used in critical functions such as those used in servo mechanisms should all have 100% QC. Most companies don't do this as it is very expensive.

Another problem resulting in failure is maintenance error or mishandling either during manufacture or during component maintenance.

For further clarification of this problem it is suggested that you read the entire thread of Mriya225


------------------
The Cat

[This message has been edited by Lu Zuckerman (edited 13 November 2000).]

[offshoreigor]

Come on people, there must be a little more on the subject out there!

If not with regards to this check, how about some comments on other 76 procedures. I'm sure you've all seen your fair share of differences!

Cheers, OffshoreIgor http://www.pprune.org/ubb/NonCGI/eek.gif



[This message has been edited by offshoreigor (edited 17 November 2000).]

[rotaryx]

Offshoreigor
I find your complacency worrying, Impossible is not a word to be used.
One nice little blob of water, not unheard of in recent weeks, runs down into the plug of either module pressure switch which is fitted in a vertical position with the plug end pointing up. Or it could be that old old Kapton wiring is starting to crack.
Result - either light or both lights.
Which system is failed ?.
This can happen when your cruising along without touching switches or when you do your servo checks. Be very sure which system you switched off first when doing the checks and if it happens in the air which cb's you pull trying to decide if its the servo jam switches or the module pressure.
unlikely, hopefully but not impossible, it has happened.

[offshoreigor]

Rotaryx:

I stand corrected. Never say never. The situation you describe is possible, but not probable. Water getting into the pressure switch? Possible, this has happened in other systems. As to the probability, how many dual servo failures have you heard of in a 76?

As for which CB to pull, I don't know how you operate but 99% of the rest of don't go past the servo jam CB's in flight.

As for this topic, we were only discussing the differences among operators in the daily interlock check on the first start of the day.

Finally, complacent? Possible, but not probable.

Cheers, OffshoreIgor http://www.pprune.org/ubb/NonCGI/eek.gif